Because a misfire in a cylinder of an engine causes a deterioration of an engine power and an increment of a rational fluctuation of the engine, it is important to detect a misfire precisely. Thus, a various types of misfire detector have been provided. The rational fluctuation of the engine represents an inevitable and periodical fluctuation of an angular speed ω of a crankshaft, which is referred to as an angular speed ω. Combustion of a fuel generating a rotational torque of the crankshaft causes the fluctuation of the angular speed ω.
A conventional misfire detector detects the misfire by measuring a rotational-varying amount E. The rotational-varying amount E is defined by a difference between a maximum value ωmax of the angular speed ω and a minimum value ωmin in a present cylinder, or a difference between a maximum value ωmax and a minimum value ωmin in a next cylinder in which fuel combustion is carried out succeeding to the present cylinder. Alternatively, the rotational-varying amount E is defined by the maximum value ωmax of the angular speed ω, or the minimum value ωmin itself.
The misfire detector determines whether the misfire has occurred, comparing a variation amount of the rotational-varying amount E with a threshold. The angular speed ω is detected by a crank angle detecting means which outputs an electric signal, which is referred to as a crank angle signal hereinafter. The crank angle detecting means comprises a subject portion and a detecting portion. The subject portion is provided on an end of crankshaft, and the detecting portion detects the rotational position of the subject portion in order to output the crank angle signal.
The conventional subject portion of the crank angle detecting means is a circular metal plate having protrusions at a predetermined angular interval on an outer circumference thereof. As shown in FIG. 6A, since pulse signals of the crank angle signal are generated every 30° CA of the crank angle θ, the detected values of the angular speed ω are indicated every 30° CA of the crank angle θ as shown in FIG. 6B.
JP-9-32620A shows that the maximum value ωmax and the minimum value ωmin of the angular speed ω are derived by applying the detected angular speed ω to a predetermined crank angle θ. For example, in the case of a four-cylinder engine, the maximum values ωmax are the detected angular speed when the crank angle θ is respectively 30, 210, 390, and 570° CA, and the minimum values ωmix are the detected angular speed when the crank angle is respectively 120, 300, 480, and 660° CA. Thus, relatively large differences between the detected values ωmax, ωmix and actual values results in a deterioration of the reliability in detecting the misfire.
Furthermore, the circular metal plate of the crank angle detecting means has a missing tooth portion on the outer circumference thereof to detect a reference position for counting the number of pulse signals. As shown in FIG. 6A, the crank angle signal in the crank angle θ corresponding to the missing tooth portion is changed into the pulse signal that requires a longer time than the other crank angle signals are changed. As shown in FIG. 6B, the detected values of the angular speed ω rapidly falls at the crank angle θ (120 and 480°0 CA) corresponding to the missing tooth portion, and then rapidly rises at the succeeding crank angle θ (150 and 510° CA) in which the angular speed is measured. Thus, the difference between the derived values and the actual values in the maximum value ωmax and the minimum value ωmin are increased in the crank angle θ affected by the missing tooth portion.